1. Field of the Invention
The present invention relates to a digital data transmission system (e.g., at 100 Mb/s) with a device for correcting the baseline wander, and is particularly suited for an ethernet system. The present invention also relates to a receiver of a digital data transmission system (e.g., at 100 Mb/s).
2. Description of Related Art
Typically, an ethernet system comprises several stations which transmit and receive data from at least one central hub by utilizing fibers or twisted wire pairs as transmission means. The several stations transmit digital data through the transmission means and both the stations and the central hub must have suitable transformers for receiving and transmitting data.
In the case where a 100 Mb/s data transmission must be achieved (for example through a category 5 wire twisted pair), a 4-bit or 5-bit encoder can be utilized. The resulting 125 Mb/s binary signal is converted to a three-level transmission signal by utilizing an MLT3 line encoding. Such MLT3 encoding results in a base band signal that consists of three levels: +1, 0, −1. A binary signal is transformed into an MLT3 signal by mapping each digital one into a transition and each digital zero into a no transition. For example, a binary series of five ones cause an output which cycles through three levels in the following order: 0, +1, 0, −1, 0.
The non-ideal transmission channel, which comprises the transformers that couple both the stations and the central hub to the transmission means, may cause the waveform of the received signal to be significantly different from the waveform of the transmitted signal. An undesired effect of the transmission channel on the waveform of the transmitted signal is commonly called “baseline wander”. To minimize the tones in the transmitted spectrum, the digital data are scrambled prior to being encoded according to MLT3 encoding. In the worst case, the output of the scrambler may have up to 56 consecutive zeroes.
The scrambled digital signal is MLT3 encoded and it is then coupled for example to a category 5 wire twisted pair through transformers. Since the transformers are high pass filters in nature, the energy below their cutoff frequency of about 50 kHz is lost. If in the signal many periods with few transitions are present, the signal loses significant energy at low frequencies due to the presence of the transformer. In this way, the received waveform can suffer clipping within the receiver and induce errors in the received data even in the case where the lengths of the lines are short. For this reason, the baseline wander effect must be corrected in the receiver.
One recent solution allows recovery of data through data slicers, that is elements able to recover data in the receiver and which allow by means of a digital-analog converter to add the information of the low frequency component to the input of the receiver, as described in the article “A CMOS Transceiver for 10-Mb/s and 100-Mb/s ethernet” (IEEE Journal of Solid-state Circuits, Vol. 33, No. 12, December 1998, pages 2169–2177).